Please refer to FIG. 1, which is an embodiment of a typical flatbed optical scanner 1 commonly seen in current market. Its main structure is that a document window glass 12 is arranged at the upper surface of the casing 11 of a scanner 1 for supporting a document to be scanned (not shown in the figure), and an optical chassis 14 is brought along by a driving device 13 to proceed linear motion along the direction of a guiding rod 15 in the hollow casing 11, such that an image scanning job is executed to the document placed on the glass 12.
Please refer to FIG. 2, which is the A-A cross-sectional view of the optical chassis 14 of an optical scanner 1 of the prior art shown in FIG. 1. The optical chassis 14 includes: a hollow casing 141, a light source 142 positioned at an appropriate position on the upper surface of the casing 141, a light-guiding device comprised by plural reflective mirrors 143, a lens set 144, and a charge coupled device (CCD) 145. The light source 142 first emits light toward the document (not shown in the figure) placed on the glass 12. After the reflected light entering the casing 141 of the optical chassis 14, it is reflected and directionally changed again by plural reflective mirrors 143 of the light-guiding device for increasing its optical length to an appropriate length and, focused by the lens set 144, the reflected light is formed as an image on the charge coupled device 145 that converts the scanned image into electronic signals, and the total track (abbreviated as TT) needed for focusing a clear image is just equal to the total value of Y1+Y2+ . . . +Y6 shown in FIG. 2.
Since the lens set 144 of the prior optical chassis 14 shown in FIG. 1 and FIG. 2 is comprised of the devices, such as convex lens, etc., so the element, the structure, and the assembly all are complicated, and its manufacturing cost is higher, and the lens set 144 will generate light-color separating effect too, such that the scanning quality is influenced and the manufacturing cost is also caused to be increased substantially. All these problems are anxiously to be solved by the relative businesses that manufacture the product of optical scanner.